Field of Invention
The present invention relates to a lock-up device, particularly to a lock-up device for transmitting torque and for absorbing and attenuating torsional vibration.
Background Art
Various devices are installed in a drivetrain of a vehicle to transmit power generated in an engine. A lock-up device can be exemplified as this type of device. A vibration reducing mechanism is used for this type of device to attenuate vibration (see e.g., Japan Laid-open Patent Application Publication No. JP-A-2011-179515).
First, in the vibration reducing mechanism, when each pair of first coil springs begins to be compressed, torsional vibration is absorbed and attenuated in accordance with torsional stiffness of each pair of first coil springs (a first stage torsional characteristic). Next, when each pair of first coil springs and each pair of second coil springs begin to be compressed, torsional vibration is absorbed and attenuated in accordance with torsional stiffness of each pair of first coil springs and each pair of second coil springs (a second stage torsional characteristic). Finally, when coiled parts of one of each pair of second coil springs are closely contacted in a condition that each pair of first coil springs and each pair of second coil springs are compressed, torsional vibration is absorbed and attenuated in accordance with torsional stiffness of the first coil springs and the other second coil spring that are still compressible (a third stage torsional characteristic).